Light-emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light.
The conversion efficiency of individual LEDs is an important factor in addressing the cost of high power LED light sources. The conversion efficiency of an LED is defined to be the electrical power dissipated per unit of light that is emitted by the LED. Electrical power that is not converted to light in the LED is converted to heat that raises the temperature of the LED. The light conversion efficiency of an LED decreases with increasing current through the LED.
LEDs are typically powered from a DC power source or a modulated square wave source so that a constant current flows through the LED while the LED is “on”. The current value is set to provide high conversion efficiency. In light sources with variable intensity, the intensity of the light is controlled by changing the duty factor of the modulated square wave so that the current flowing through the LED is at a value consistent with providing the desired efficiency.
Conventional lighting systems for use in buildings typically must be powered from an AC power source. Hence, an LED-based replacement light source typically includes an AC-DC power converter. The cost of the power converter represents a significant fraction of the cost of a typical LED light source. In addition, the power losses in the power converter reduce the overall efficiency of the light source. In addition, such AC-DC converters are not as reliable as that of LEDs, and hence, can limit the lifetime of the lighting system.
To avoid these costs, LED light sources that operate directly from an AC power source without the power first being converted to DC have been proposed. For example, light sources that include two strings of LEDs have been proposed. The LEDs are connected in series in each string. One string is powered on when the AC waveform is in the positive half of the sine wave, and the other is powered when the AC waveform is in the negative half of the sine wave. This simple driving scheme suffers from low efficiency and flicker. To improve the efficiency, light sources that include a full-wave rectifier have been proposed; however, such light sources still have low efficiency and exhibit flicker.
Consider a single LED that is driven by an AC waveform. In general, the LED is characterized by a turn-on voltage, Vf, which must be exceeded to forward bias the LED so that a substantial current will flow through the LED. The LED will remain off until the sine wave reaches this voltage. When the voltage is greater than this turn-on value, the LED will generate light; however, the voltage drop across the LED must also be maintained below a maximum value, Vd, at which the LED will be damaged. In general, the current through the LED increases exponentially with voltage above the turn-on voltage until the current is limited by the series resistance of the LED. Hence, the difference between the turn-on and maximum voltages that characterize the allowable operating range of the LED is relatively small. For example, Vf is approximately 2.75V and Vd is approximately 3.6V for GaN blue LEDs. Vf is determined by the dominant wavelength of the emitting light. Vd is determined by the overall heat consumption the packaged LEDs are capable of enduring or the highest current density allowed to the LEDs without causing long term reliability issues.
To accommodate the maximum voltage, Vs, of a typical building power source, a number of LEDs must be connected in series. The minimum number of diodes must be greater than Vs/Vd to prevent damage to the LEDs unless a current limiting mechanism is included in the drive circuitry which consumes further power. For example, with the 110V AC system, the peak voltage is 156V, i.e., Vs=156V, approximately 43 LEDs must be placed in series to withstand the peak voltage. However, the string will cease to make light when the voltage drops to 118V. As a result, light is generated approximately 30 percent of the time. This leads to a 120-cycle flicker. In addition, the number of LEDs that must be used to generate a predetermined average light intensity is more than three times the number needed in a DC driving scheme, which increases both the component and the packaging costs.
In a co-pending application, U.S. Ser. No. 12/504,994, filed on Jul. 17, 2009, an improved AC LED light source is described in which each LED in a series string is connected in parallel with a switch that shorts that LED when the AC voltage across the string is insufficient to drive all of the LEDs in the string. By removing LEDs from the string when the AC voltage is below the voltage needed to drive all of the LEDs, the duty cycle is substantially increased. However, the resulting light intensity varies approximately sinusoidally. In addition, the light source will still cease to make light when the AC voltage falls below Vf. This “dark” period further increases the perception of a flickering source. Hence, the flicker problem remains. In addition, the average number of LEDs generating light over the cycle is still substantially less than 100 percent. Finally, the cost of the light source is increased by the number of switches needed to implement this scheme.